Survives for months in lava

Then, finally, the great beast could take no more.  His last defense against total extinction took over.  With a grunt, Godzilla closed his eyes and lost consciousness, as all his biological systems shut down.  As Godzilla fell into a kind of deep hibernation, he toppled.
With an enormous splash, the creature plunged into the lava.  His still-living body was quickly swept along by the flow of the underground river.  For days or weeks or months, Godzilla’s carcass drifted on the river of lava.  Molten metal burned at his hide, eating away at his protective outer layer of flesh.
If things had continued this way for a little while longer, Godzilla might have perished there, his death unlamented.